The present invention relates generally to Code Division Multiple Access (CDMA) systems, and more particularly, to channel quality estimation for High Speed Downlink Packet Access (HSDPA) in Wideband CDMA (WCDMA) systems.
HSDPA is a wireless communication protocol for WCDMA systems specified by the Third Generation Partnership Project (3GPP). HSDPA provides a method for delivering packet data to a plurality of mobile terminals over a shared downlink channel called the High Speed Downlink Shared Channel (HS-DSCH). HS-DSCH is divided into successive timeslots or scheduling intervals, which are called Transmission Time Intervals (TTI) in the standard. During a given TTI, one or more mobile terminals may be scheduled to receive packet data from a base station on the HS-DSCH. The mobile terminals sharing the HS-DSCH estimate the channel conditions and report the channel quality estimates to the base station on an uplink channel called the High Speed Dedicated Physical Control Channel (HS-DPCCH). A scheduler at the base station uses the channel quality estimates provided by the mobile terminals to schedule transmissions to the mobile terminals. The channel quality estimates are also used to select a modulation and coding scheme for the scheduled mobile terminal.
In order to achieve higher data rates, it has been proposed to use multiple transmit antennas at the base station to transmit packet data to the mobile terminals, which may have one or more receive antennas. Such systems are referred to as multiple-input, single-output (MISO) and multiple-input, multiple-output (MIMO) systems. Multiple antenna systems exploit the spatial dimensions of the signal at the receiver to achieve higher spectral efficiency and higher data rates without increasing bandwidth. In multiple antenna systems, there is typically no one antenna configuration that performs consistently better than the others over the range of possible operating conditions envisioned for deployment. Therefore, antenna configuration selection may be employed to select the antenna configuration that is best suited for the currently-scheduled mobile terminal.
When antenna configuration selection is employed in a multiple antenna system, estimating channel conditions can be problematic for mobile terminals that are not currently-scheduled, i.e., the inactive mobile terminals. The inactive mobile terminals must estimate channel conditions as if they were currently-scheduled. However, the antenna configuration selected for the currently-active mobile terminal may not be the most desirable configuration for the inactive mobile terminal. Moreover, the inactive mobile terminals typically do not have knowledge of the antenna configuration being used. Such knowledge is necessary in order to remove the effects of the current antenna configuration from the calculation of channel quality estimates.
U.S. patent application Ser. No. 11/449,258 filed Jun. 8, 2006 describes a method of estimating channel quality in which the antenna configuration is first estimated by the inactive mobile terminals. The estimated antenna configuration is then used to remove the effects of the antenna configuration for the currently-scheduled mobile terminal from the channel quality estimates reported to the base station. The mobile terminals can then report channel quality assuming an antenna configuration that is best for them. While this technique is useful in some operating conditions, errors in channel quality estimation may nevertheless result.
Improvements in the reliability of channel quality estimates would result in greater spectral efficiency. Therefore, an improved method of channel quality estimation for multiple antenna systems is needed.